MediaRouter: Add SystemMediaRoute2Provider to publish default route

SystemMediaRoute2Provider provides media routes for local playback.
For now, it only publishes two routes; phone speaker (default route)
and a2dp bluetooth route.
When we can get multiple blutooth devices, this will publishes those.

Test: atest mediaroutertest (failed tests are not related to this CL)
Change-Id: I4193b5ad1669eb09c97218a0d1fb6d1695010cdd